Motor and electrical apparatus housing same

ABSTRACT

A motor of this invention comprises a rotor having a permanent magnet, the number of which magnet poles is P, and a stator including M pcs of teeth, the teeth arranged in a circumferential direction in a manner to face the permanent magnet through a spatial gap, wherein the stator includes stator core having the number M of the teeth, and a winding wire wound about each of the tooth, wherein the number P of the magnet poles and the number M of the teeth have a relation defined by formulae (2/3)M&lt;P&lt;(4/3)M, and M#P, and wherein ratio (t1/Ds) of teeth tip width t1 to stator inside diameter Ds is given by a formula 0.18&lt;(t1/Ds)&lt;0.25.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/636,303, filed Sep. 20, 2012 and which is national stage entry ofPCT/JP2011/001720, filed Mar. 24, 2011, which claims the benefit ofJapanese Patent Application No. 2010/069616, filed on Mar. 25, 2010, thedisclosures of which are incorporated herein in its entirety byreference.

TECHNICAL FIELD

This invention relates to a brushless motor which employs a permanentmagnet.

BACKGROUND ART

A permanent magnet brushless motor in which a concentrated winding wireis made on every tooth thereof is widely used in an electrical homeapparatus, audio equipment, an information apparatus, transportationequipment and the like. With this type of motor, torque ripple orcogging torque occurs with magnetic attraction exerting over permanentmagnet and teeth, causing a vibration and a noise problem.

To obtain a low vibration and a low noise motor, methods of reducing thecogging torque have been practiced, such as optimization of number ofpoles and teeth, skewing, and magnetization of the magnet in sinusoidalwaveform. For examples, patent document 1 discloses a method of reducingcogging torque by determining a relation between number of pole P andnumber of teeth M to P:M=10:12. Patent document 2 discloses a method forreducing torque ripple by determining a relation between number of poleP and number of teeth Q to P/Q>0.8, and a relation between teeth tipwidth bt1 and pole pitch τρ to bt1/τρ≦0.8. Patent document 3 discloses amethod of reducing the cogging torque, with a motor having a P to Qratio of 3:4, by determining a teeth width at an electrical angle of 145to 165 degrees and 85 to 105 degrees, without sacrificing an outputtorque. Patent document 4 discloses a structure in which a shape of amagnet arranged on a surface of rotor is made thicker toward a center ofthe magnetic pole but gradually made thinner toward a space betweenmagnetic poles. By taking the structure, patent document 4 aims apattern of magnetic flux density on the surface of the rotor to comecloser to a sinusoidal waveform, thereby reducing cogging torque andachieving a low vibration and a low noise. Patent document 5 describesan idea of magnetizing the magnet in essentially a sinusoidal waveforminstead of adjusting configuration of the magnet, aiming a surfacemagnetic flux waveform forms a lean sinusoidal waveform and a surfacemagnetic flux in a neighborhood of pole boundaries becomes essentiallyzero. By magnetizing the magnet in this pattern, patent document 5 aimsto decrease cogging torque and therewith realize a low vibration and alow noise.

However, with above mentioned conventional technologies, althoughcogging torque or torque ripple causing noise and vibration is somewhatreduced, motor efficiency is not yet specifically addressed. Theconventional technologies are therefore not complete enough as a motorfor home apparatus such as a fan motor of air conditioner where demandnot just for a low vibration and a low noise but for a high efficiencyis increasing year by year, leaving a task. Further, adjusting the shapeof the magnet as in patent document 4 is difficult in production.Furthermore, for securing a large magnetic flux density, a thick magnetis required, increasing a usage amount of the magnet therefore anincrease in cost, leaving a task. Still further, with the magnetizingmethod described in patent document 5, a neighborhood of pole boundariesis not fully magnetized, so magnetic power of the magnet is noteffectively utilized, lowering the efficiency of the motor, leavinganother task.

With a conventional fan motor for air conditioner, a link type dividedcore is traditionally used in which divided cores are linked together toenhance a motor efficiency and to increase production efficiency. Thisstructure allows a winding nozzle to fully utilize a nozzle passingspace, making an aligned winding of wire possible and therefore a highdensity winding possible. The increased winding enhances torque,decreasing cooper loss and so increasing efficiency of the motor.However, with such a configuration, a minute gap may be created whendivided cores are linked together, the gap space decreasing torque,leaving a task. Still further, when linking the divided cores, adimensional difference is likely to appear in an inside periphery of thedivided core because of a work precision of the divided core andassembling error of the core, creating an uneven gap distributionbetween the stator and the rotor, making magnetic variation large, hencecausing the vibration and the noise large, leaving still other task.

RELATED ART LITERATURES Patent Literature

PTL 1 Japanese Patent Gazette No. 2954552

PTL 2 Japanese Patent Unexamined Publication No. 2001-157428

PTL 3 Japanese Patent Unexamined Publication No. H08-322167

PTL 4 Japanese Patent Unexamined Publication No. H06-217478

PTL 5 Japanese Patent Unexamined Publication No. 2003-111360

SUMMARY OF THE INVENTION

A motor of this invention comprises a rotor having a permanent magnet,the number of which magnet poles is P, and a stator including M pcs ofteeth, the teeth arranged in a circumferential direction in a manner toface the permanent magnet through a spatial gap, wherein the statorincludes stator core having the number M of the teeth, and a windingwire wound about each of the tooth, wherein the number P of the magnetpoles and the number M of the teeth have a relation defined by formulae(2/3)M<P<(4/3)M, and M≠P, and wherein ratio (t1/Ds) of teeth tip widtht1 to stator inside diameter Ds is given by a formula 0.18<(t1/Ds)<0.25.

Having this structure, cogging torque is reduced and a motor of a lowvibration, a low noise and a high efficiency is provided withoutaccompanying a cost increase or an efficiency reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of a motor according to first exemplaryembodiment of the invention.

FIG. 2 is a cross section view of a stator core of the motor accordingto a first exemplary embodiment of the invention.

FIG. 3 is a comparison chart illustrating a cogging torque of the motoraccording to the first exemplary embodiment of the invention and that ofa motor for comparison.

FIG. 4 is a graphical chart illustrating analytical results of arelation between efficiency of the motor of the embodiment and ratiot1/Ds of the same.

FIG. 5 shows a surface magnetic flux waveform of a permanent magnetaccording to the first exemplary embodiment of the invention.

FIG. 6 is a structural drawing of electrical equipment according to asecond exemplary embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the invention are explained byreferring to drawings.

First Embodiment

FIG. 1 is a cross sectional view of motor 10 according to firstexemplary embodiment of the invention. FIG. 1 is a cross section viewedfrom a longitudinal direction of a rotational shaft. With this exemplaryembodiment, an inner rotor type brushless motor is explained.

As FIG. 1 shows, motor 10 of this embodiment comprises stator 11 androtor 20. Stator 11 includes stator core 12 which winding wire 13 windsabout. Rotor 20 is rotatably positioned inside an internal circumferenceof stator 11.

Stator core 12 includes ring-shape yoke 14 and a plurality of teeth 15extruding from an internal circumference of yoke 14 in which the teethare arranged along a circumferential direction with equal intervals. Ata tip of each tooth 15, wide portion 15 a is formed, the wide portionbeing broadened along the circumference. An opening slot 16 is formedinside an internal circumference of yoke 14 and in-between adjacentteeth 15. Winding wire 13 is wound about every tooth 15 utilizing theopening space of slot 16. FIG. 1 shows only one winding wire 13 is woundabout tooth 15, but winding wire 13 is wound about every tooth 15concentratedly, forming what is called there-phase winding U, V and W.In this embodiment, number of teeth 15 or number of slot 16 is 12.

Stator core 12 in the embodiment is not a conventional divided corelinked type but the stator core is integrated into the stator, makingone piece. Further, stator core 12 is formed by laminating a pluralityof magnetic thin plates in a direction of thickness thereof. By takingthis structure, the spatial gap between the divided cores is eliminated,there is no torque reduction and an output power is larger than dividedcore type. In this way, the embodiment takes advantage the structure ofthe stator core integrated one piece stator, improving the efficiency ofthe motor. Since an effect of production error such as an erroneousinside diameter of the divided core is greatly removed with thisstructure, cogging torque is lowered and thereby a high efficiency, alow noise and a low vibration motor is achieved.

Rotor 20 includes permanent magnet 22 having a plurality of magnet polesdisposed on an outer peripheral side of rotor core 21 in which the Npole and the S pole are alternately placed at an identical interval.Rotor 20 is structured to hold permanent magnets 22 pole by pole, or tohold a cylindrical ring type magnet. Rotor 20 is placed so as permanentmagnet 22 to face teeth 15 through a certain space that is createdinside peripheral side of teeth 15. In this exemplary embodiment,permanent magnet 22 has 10 magnetic poles.

Rotor 20 having rotor core 21 connected to rotational axis 24 is held ina rotatable manner with regard to a center of rotational axis 24, sothat rotor 20 rotates in the peripheral direction with facing stator 11.As described, motor 10 of this embodiment has number of magnetic polesP=10 and number of teeth M=12, namely it is 10 poles and 12 teethstructure.

With this structure, when an alternate current is applied to windingwire 13 of stator 11, magnetic attractive force and repulsive force aregenerated between permanent magnet 22 and teeth 15. With the attractiveforce and the repulsive force, rotor 20 rotates about rotational axis24.

Next, structural details of stator core 12 of the embodiment aredescribed.

FIG. 2 is a cross section view of stator core 12 of the embodiment. Inthe embodiment, as is in FIG. 2, Ds which is an inside diameter ofstator core 12 and t1 which is a width of a tip of teeth 15 or adimension of tooth tip wide portion 15 a in the peripheral direction areboth specified to satisfy a prescribed relationship. In the invention, aratio (t1/Ds) of teeth tip width t1 to stator inside diameter Ds isspecified to be 0.18<(t1/Ds)<0.25. Namely, (t1/Ds) is determined withina range larger than 0.18 and smaller than 0.25.

Following, details of the embodiment is explained. As mentioned, theinvention aims to provide a motor which has a reduced cogging torque andof which efficiency reduction is being controlled.

First, in order to reduce the cogging toque, motor 10 is structured with10 magnetic poles and 12 teeth. Cogging torque is defined to be a rippleof torque at no power application. Cogging torque is caused with achange in permeance (an inverse of magnetic resistance) between statorslots and a rotor, and it is a least common multiple of number of slotsand number of magnetic poles per one rotational cycle. When the leastcommon multiple is large, the change in permeance per ripple is small.So, as larger is the least common multiple, the smaller is the coggingtorque. In this embodiment, 10 poles 12 teeth structure is selected sothat the least common multiple be large (the least common multiple is60), and that cogging torque be minimum.

In the embodiment, 10 poles 12 teeth structure is exemplary explained,but number P of magnetic poles and number M of teeth may be determinedby using formulae of (2/3)M<P<(4/3)M and M≠P, achieving a similar effectas to the exemplary embodiment.

In FIG. 3, cogging torque of motor 10 of the exemplary embodiment and ofa comparison motor is compared. In FIG. 3, motor 10 of the exemplaryembodiment has 10 poles 12 teeth, and the comparative motor has 8 polesand 12 teeth (which least common multiple is 24). The horizontal axis(t1/Ds) in FIG. 3 indicates a ratio between teeth tip width t1 andstator inside diameter Ds, and the vertical axis indicates amplitude ofcogging torque with regard to varying (t1/Ds) ratio. The graph showsanalytical calculation results of relation between the amplitude of thecogging torque and the (t1/Ds) ratio. Shown in FIG. 3 are values ofcogging torque when ratio (t1/Ds) is varied between 0.16 and 0.26. Thecomparative motor having 8 poles 12 teeth is ordinarily used as abrushless motor for a fan motor of air conditioner, for instance.

As is evidently shown in FIG. 3, when the ratio (t1/Ds) of motor 10 ofthe embodiment and the ratio (t1/Ds) of the comparative motor are samevalue, the cogging torque of motor 10 of the embodiment is smaller thanthat of the comparative motor. It is also indicated that, when the ratio(t1/Ds) changes between 0.16 and 0.26, cogging torque of motor 10 of theembodiment changes two times, while that of the comparative motorchanges four times as much. In the fan motor of air conditioner, it isacknowledged that noise caused by cogging torque is allowed as long ascogging torque is below about 2 mNm, therefore cogging torque in(T1/Ds)>0.175 is especially preferable. On the other hand, coggingtorque of the comparative sample becomes larger than 2 mNm at a similar(t1/Ds) zone of 0.175<(t1/Ds)<0.25, namely 5 times as larger thanembodiment sample at the (t1/Ds) ratio of 0.19. It means that thecomparative motor hardly realizes a low ripple or a low noise at thezone of 0.175<(t1/Ds)<0.25.

Above comparison proves that a motor having a smaller cogging torque isobtained by employing 10 poles 12 teeth structure. It also proves that,when structuring motor having 10 poles 12 teeth like motor 10 of theembodiment, the (t1/Ds) ratio is preferred to exceed 0.175 to reduce theeffect of cogging torque.

Second, with the embodiment, a structure of preventing reduction ofmotor efficiency is explained.

It is generally known that copper loss and iron loss are factorsaffecting efficiency loss of a motor.

First, copper loss Wcu is in a proportional relationship with a squarevalue of current I and resistance value R. Specifically, the relationbetween copper loss Wcu, current I and resistance value R is expressedby a formula of Wcu=RI². It means, copper loss Wcu changes with a changein a square value of current I, as long as copper wire is identical.Meanwhile, motor torque T is ordinarily proportional to an amount ofmagnetic flux φ and current I. In this situation, if teeth wide portion15 a is broadened, an opposing dimension of the wide portion topermanent magnet 22 increases, consequently increasing an amount ofmagnetic flux φ to stator core 12. It means, when torque T is unchangedand teeth wide portion 15 a is made larger, motor current I becomessmaller; the wider becomes teeth wide portion 15 a, the smaller becomescopper loss Wcu.

Second, iron loss Wfe is in a proportional relationship with magneticflux density B and rotating speed f. As long as rotating speed f isunchanged, iron loss Wfe changes as magnetic flux density B changes. Ifteeth wide portion 15 a is larger, it becomes easier for the portion toabsorb more magnetic flux, increasing magnetic flux density B. Sinceiron loss Wfe is in the proportional relationship with magnetic fluxdensity B, the wider is teeth wide portion 15 a, the larger becomes ironloss Wfe.

Efficiency of a motor is thus dependent on an integrated loss of thecopper loss and the iron loss. Copper loss is dominant when teeth wideportion 15 a is narrow, and iron loss becomes dominant when teeth wideportion 15 a is broad.

With the exemplary embodiment, a magnitude of teeth tip width t1 andinside diameter Ds are specified so that the ratio (t1/Ds) between teethtip width t1 and inside diameter Ds fall within a specified range. Whenstator inside diameter Ds is increased, diameter of rotor 20 may beincreased, increasing an amount of magnetic flux from permanent magnet22 to stator core 12, allowing teeth tip width t1 to be increased. Thus,when stator inside diameter Ds is increased, the dimension of teeth wideportion 15 a opposing to the permanent magnet may be increased forsuppressing the copper loss. Meanwhile, if teeth tip width t1 isincreased as stator inside diameter Ds increases, magnetic flux tostator core 12 is increased, increasing the iron loss. As such, if theratio (t1/D) is set in an appropriate value, the iron loss and thecopper loss may both be well controlled, realizing an optimal motorefficiency.

FIG. 4 is a graphical chart illustrating analytical results of arelation between motor efficiency and ratio (t1/Ds) of motor 10 of theembodiment.

As shown in FIG. 4, a highest efficiency is achieved when the ratio(t1/Ds) is around 0.215. The motor efficiency falls down from the peakas the ratio (t1/Ds) becomes smaller and the ratio (t1/Ds) becomeslarger as well.

In the air conditioner fan motor market where competition in productefficiency is keen, it is generally known that efficiency fall down by0.2% or more means a significant regression in performance. Therefore,with the embodiment, the zone of 0.18<t1/Ds<0.25 is particularlypreferable in which reduction in motor efficiency is at most within 0.2%from the highest point.

As described, with the preferred embodiment, cogging torque is keptsmall with 10 poles 12 teeth structure. Furthermore, the ratio (t1/Ds)set within a zone larger than 0.18 and smaller than 0.25 prevents motorefficiency to fall down.

Winding wire 13 wound about teeth 15 has preferably a space factor of atleast 60%. As long as the winding space factor of winding wire 13 woundon stator 11 is 60% or more of the slot dimension, a performance equalto or better than an identical divided core type motor is achieved.

Stator 11 of motor 10 is preferably resin molded. Resin molded, rigidityof the stator is enhanced and magnitude of resonant vibration of themotor is reduced, securing a higher efficiency, a lower noise and alower vibration motor.

With the embodiment, a cylindrical ring type permanent magnet 12 isexplained. The magnet is not limited to the ring type and other type ofmagnet such as a segment magnet may be used. Yet, since the rotor ofring type magnet is composed of a single magnet, installation is easyand reliability is high, help reducing an assembling cost. In addition,the ring type magnet has a smaller dimensional difference betweenmagnetic poles and has a smaller magnetic unbalance due to an assemblingerror, so that a low cost, a low noise and a low vibration motor isprovided than by using a plurality of segment magnets. Thus, the ringtype permanent magnet is preferred.

Permanent magnet 22 is preferred to be a bonded rare earth magnet. Sincebonded rare earth magnet has a stronger magnetic force than a sinteredferrite magnet, an output is high and a miniaturized high efficiencymotor is becomes available.

With permanent magnet 22, surface magnetic flux is preferred to bemagnetized forming substantially a rectangular waveform in thecircumferential direction of the rotor. As shown in FIG. 5, when thesurface magnetic flux is magnetized to form the rectangular waveform inthe circumference direction, the stator receives a larger amount ofmagnetic flux than when it is in a sinusoidal waveform. The stator thusefficiently utilizes the magnetic force, and therefore the efficiency ofthe motor is further enhanced.

By installing above structured motor in an electric home apparatus, alow vibration, a low noise and a high efficiency apparatus is realized.

As has been explained, with the motor of the invention, the relationbetween the number P of poles and the number M of teeth is defined byformulae (2/3)M<P<(4/3)M, and M≠P, with which cogging torque is keptlow. Further, with the motor of the invention, the ratio (t1/Ds) betweenthe teeth tip width t1 and the stator inside diameter Ds is given by theformula 0.18<(t1/Ds)<0.25, maintaining the high efficiency of the motor.Thus, the cogging torque of the invention is reduced, and the highefficiency motor with little efficiency fall down is realized.

Second Embodiment

A structure of an indoor equipment of an air conditioner as an electricapparatus of second embodiment of the invention is explained first.

As in FIG. 6, motor 201 is installed inside housing 211 of indoorequipment 210. Cross flow fan 212 is attached to a rotational axis ofmotor 201. Motor 201 is driven by driving mechanism 213. Powered bydriving mechanism 213, motor 210 rotates, turning cross flow fan 212.Rotating cross flow fan 212 sends out air which is conditioned by anindoor heat exchanger (not illustrated) into a room. In this case, motor210 may be motor 10 of the first exemplary embodiment.

The electric apparatus of the invention includes the motor and thehousing installing the motor, the motor being in above mentionedconfiguration of the invention.

Above, a motor installed in an indoor unit of an air conditioner isexplained as an exemplary embodiment of electric apparatus of theinvention. This motor may be applied to an outdoor unit of airconditioner, a water heater and a washing machine. It may also beapplied to a variety of information apparatus and industrial apparatus.

INDUSTRIAL APPLICABILITY

With the motor of the invention, cogging torque is reduced and anefficiency fall down of the motor is prevented, so that a low vibration,low noise and high efficiency motor is provided. The motor is suitablyapplied to electric home apparatus where a low vibration, a low noiseand a high efficiency are required.

What is claimed is:
 1. A motor comprising; a rotor having a permanent magnet, the number of which magnet poles is P; and a stator including teeth, the number of which is M, the teeth arranged in a circumferential direction in a manner to face the permanent magnet through a spatial gap, wherein the stator includes a stator core having the number M of the teeth, and a winding wire wound about each of the teeth, wherein the number P of the magnet poles and the number M of the teeth have a relation defined by formulae (2/3)M<P<(4/3)M, and M≠P, and wherein ratio (t1/Ds) of teeth tip width t1 to stator inside diameter Ds is given by a formula 0.18<(t1/Ds)<0.25.
 2. The motor as listed in claim 1, wherein the stator core is structured by laminating a plurality of magnetic thin plates in a direction of thickness thereof.
 3. The motor as listed in claim 1, wherein the winding wire wound about the teeth has a space factor of at least 60%.
 4. The motor as listed in claim 1, wherein the stator is molded with a resin.
 5. The motor as listed in claim 1, wherein the permanent magnet is a ring type magnet.
 6. The motor as listed in claim 1, wherein the permanent magnet is a bonded rare earth magnet.
 7. The motor as listed in claim 1, wherein the permanent magnet is magnetized to have a surface magnetic flux of substantially a rectangular waveform in the circumferential direction.
 8. An electric apparatus equipped with the motor listed in claim
 1. 9. An electric apparatus equipped with the motor listed in claim
 2. 10. An electric apparatus equipped with the motor listed in claim
 3. 11. An electric apparatus equipped with the motor listed in claim
 4. 12. An electric apparatus equipped with the motor listed in claim
 5. 13. An electric apparatus equipped with the motor listed in claim
 6. 14. An electric apparatus equipped with the motor listed in claim
 7. 